1. Field of the Invention
The invention relates to a process for manufacturing, under pressure casting, parts in a magnesium matrix composite material.
Throughout this document, the term magnesium must be understood as also including all the magnesium based alloys.
On the other hand, the expression "magnesium matrix composite material" includes any material having a reinforcement structure, generally formed of long fibers such as carbon fibers, alumina fibers, etc., sunk into a magnesium matrix. The volume rate of the fibers contained in the material is generally included between about 40% and about 60%.
The process according to the invention can be used advantageously for manufacturing any foundry part requiring both good mechanic characteristics and a reduced mass. Preferential applications of this process can be found notably in the aeronautic and airspace industries.
2. Discussion of the Background
The pressure casting technique (in most cases between about 30 bars and about 100 bars) has been known for manufacturing metallic matrix composite material parts for some years.
According to this technique, are placed in a single hermetic container, comparable to an autoclave, a crucible containing metal blocks designed to form the matrix of the part, and a mold into which has previously been inserted a fiber preform.
During a first step, the insides of the container and of the mold are put under vacuum, the crucible containing the metal blocks is heated and the mold is pre-heated.
When the metal contained in the crucible is entirely molten, it is transferred into the mold. This transfer is executed automatically by pressurizing the container to a defined pressure level, generally comprised between about 30 bars and about 100 bars.
As soon as the mold is full, the cooling of the part is accelerated by bringing a cooling device in contact with one of the mold walls. As long as the temperature has not fallen under the solidification temperature of the metal, the pressure is maintained in the container in order to complete the natural contraction of the metal.
The main known implementation techniques of this process are described in "Pressure Infiltration Casting of Metal Matrix Composites" by Arnold J. Cook and Paul S. Werner in "Materials Science & Engineering" A 144 (October 1991) PP 189-206.
In one of these known techniques, the crucible containing the metal blocks is fixed above the mold, the higher part of which having a receptacle in the bottom of which opens the mold printing of the part to be manufactured. During its fusion, the metal flows into the receptacle through an aperture formed in the bottom of the crucible and initially sealed up. The molten metal is then transferred into the mold printing due to the pressurization of the container. Then, the part is cooled by a cooling plunger brought into contact with the bottom of the mold.
This first technique, wherein the crucible is placed above the mold, has the advantage of enabling the use of a basic and therefore relatively cheap cast. It is thus fairly inexpensive. But, this technique is hardly applicable to the manufacturing of magnesium matrix composite parts, albeit the interest offered by such parts in certain industries, such as the aeronautic and space industries. In fact, the preliminary transfer of the molten metal in the receptacle formed at the upper part of the mold is carried out under vacuum and without any particular precautions. So, the magnesium then risks to evaporate and to deposit itself throughout the installation, causing part of this installation to be non-operative. On the other hand, no precaution has been taken to avoid a magnesium/oxygen explosive reaction, especially when the enclosure is put under pressure.
According to another known technique described in the aforementioned paper of Cook and Werner and in the document EP-A-O 388 235, the crucible containing the metal blocks is fixed under the mold, the lower part of which being equipped with a supply tube, which initially opens above the crucible. The putting under vacuum is done through a vacuum tube that opens directly into the mold. When the metal is molten, the crucible is lifted so that the supply tube of the mold plunges into the molten metal. Thereafter, the transfer of the molten metal into the mold is obtained by pressurizing the container. The cooling of the part is ensured by a cooling block that is brought into contact with the upper wall of the mold.
This technique, in which the crucible is placed under the mold, is more expensive than the preceding one, since the mold must have a supply tube. Conversely, it avoids the intermediary step of transferring the molten metal.
On the other hand, this technique is also non-adapted to the manufacturing of magnesium matrix composite parts. Indeed, the fusion of the metal is entirely carried out under vacuum, as in the preceding technique, so that an evaporation of the magnesium under vacuum is almost inevitable. Furthermore, no special precautions have been taken to avoid a magnesium/oxygen explosive contact.
Moreover, in the document EP-A-O 388 235 as in the part of the above-mentioned paper related to this technique, the putting under vacuum of the container is carried out by a passage under vacuum directly opening into the mold. This results in a further increase of the mold complexity and cost. Furthermore, the liquid metal runs the risk to be sucked by the circuit under vacuum when the mold is filled. Moreover, the presence of this passage under vacuum leads to reduce the thermal exchange surface used to cool the mold during the last phase of the process.
This analysis of the existing techniques for the manufacturing of reinforced metallic parts by pressure casting shows that none of them are adapted for the manufacturing of magnesium matrix parts. Furthermore, no clear adaptation of these techniques to the manufacturing of magnesium matrix parts is suggested in the present state of the art.